The present invention relates to laser technology, and more particularly to a dielectric multilayer reflector and a laser system using such reflectors.
As resonators for a laser system, fully reflective mirrors, which completely reflect laser beams, and output reflective mirrors, which selectively take out a part of laser beams, are commonly used. These laser mirrors are given their desired reflectivity or transmissivity by coating multiple dielectric layers over a glass or quartz substrate. A dielectric multilayer reflector is usually produced by alternately vapor-depositing films of high and low refractive indices, each layer having a thickness of .lambda./4, and the materials to be vapor-deposited, design of the number of layers and vapor-depositing method are well known to those skilled in the art (for instance the U.S. Pat. No. 4,099,840). Since a reflector of this kind, including both its substrate and coated films, consists of insulative materials, its film surfaces have high resistivities both thereupon and in the vertical direction thereof, and therefore are readily electrically chargeable.
If a reflector coated in this manner is used in the path of a laser beam, the part of the reflector irradiated by the beam will be electrically charged in proportion to the duration of irradiation. The charged part will gradually attract dust and vapor present nearby or in the air, and therefore become unable to maintain its prescribed reflectivity or transmissivity. At the same time, the dust and vapor which have adhered to the reflector will absorb the laser beam and eventually destroy the vapor-deposited films. This phenomenon is particularly conspicuous where the reflector is used as a resonator because of the greatness of the laser power within the resonator.
To make up for this disadvantage, it has been necessary, where a laser mirror is used, to isolate either the laser-irradiated part or the whole surface of the mirror from the external air and to keep it dry, as well as to fill the path of the laser beam with dust-free clean air. However, even if this precaution is taken, it is impossible to perfectly shut out dust and water vapor, and therefore the laser mirror has to be cleaned from time to time while in use.